1. Field of the Invention
The present invention relates to a stepping motor used as a drive source of a recorder having an image forming function such as a facsimile device, a copying machine or a printer, and more particularly to mounting of an encoder on such a stepping motor and a dust-proof structure of the encoder.
2. Related Background Art
U.S. Pat. No. 4,963,808 discloses mounting of an encoder, on a stepping motor. In this stepping motor, an encoder having to-be-detected areas which are equal in number to an integer multiple of the number of magnet poles of a rotor, is secured to a shaft of the rotor, and the number of to-be-detected areas of the encoder is counted at a predetermined position in a stator as the rotor is rotated so that energization of coils of the stator is switched when the count reaches a predetermined value.
U S. Pat. No. 4,928,050 discloses a printer which uses such a stepping motor having an encoder. In this printer, the stepping motor is used as a drive source to drive a record head for scanning. The printer comprises detection means for detecting a rotation angle position of a rotor of the stepping motor and control means for closed-loop controlling the drive of the stepping motor in accordance with the detection output of the detection means.
In each of the above stepping motors, the encoder of the stepping motor is assembled in a dust-proof case.
In such a motor with the encoder, a sensor, such as a photo-interruptor, is positioned and fixed or a stationary mount such as a motor case, and a coded disk such as a slitted disk is positioned and fixed to a motor shaft.
Since the sensor, such as a photo-interruptor, is positioned and fixed to a mount plate secured to the motor case by fastening means such as screws, the position tolerance between the motor case and the motor shaft as well the position tolerance between the motor case and the mount plate are accumulated, so that an accumulated relative position error between the coded disk and the sensor increases, and it is difficult to attain a sufficiently high detection precision of the motor rotation position.
In such a motor with the encoder, when the coded disk such as slitted disk is to be secured to the motor shaft, a receiving member having a receiving plane for positioning the encoding disk and a center boss is press-fitted to the motor shaft, the disk is fitted to the receiving member, and a washer is coupled to the receiving member by a bolt to secure the coded disk.
In such prior art method for securing the encoder, the number of steps for securing the coded disk such as slitted disk is large, the work is complex, and the cost is high.
Further, motor terminals are connected to a motor terminal board or lead wires, encoder terminals are connected to other lead wires and they are taken out separately. Accordingly, the structure of connection of the lead wires of the motor is complex and the assembling takes a long time.
When the sensor is to be mounted on the motor case, a positioning area such as a projection or a recess is provided on a motor outer tube, and the sensor is positioned and clamped by screws. Alternatively, the sensor is mounted by using a motor mount plate and lead wires are connected to input/output terminals of the sensor. Accordingly, the costs of parts are high and the number of steps of assembling is large.
On the other hand, because the magnetic field generation means and the excitation means of the motor generate heat when the motor is driven, it is necessary to secure heat dissipation in case of continuous operation or an operation in a high temperature environment.
In a compact size motor such as a stepping motor, there is no space to accommodate a fan or cooling medium, and the heat is dissipated to the air by conducting the heat to the outside through the motor shaft or the motor case.
In the prior art compact size motor, however, a heat dissipation plate is separately mounted on the surface of the motor case to enhance the heat dissipation effect. As a result, the number of parts increases, the number of assembling steps increases, and the size of the motor increases.